4a. Q) How can I upgrade from my existing Windows install? What are the possible upgrade scenarios?
A) Upgrade Paths for Windows 8 are outlined here:
Windows 8 Upgrade Paths
Here we will briefly describe the types of installs available in a bit more detail than what is posted at Microsoft's TechNet site above. There are 3 types, and depending on what version of Windows you are starting from, you will have different options available to you.
In-Place Upgrade: This type of install is what most people think of when they think of the term Upgrade Install. This type of install preserves most Windows and application settings from your existing install, and your applications will be there when your computer boots Windows 8 for the first time.
Migration Install: This type of upgrade will backup your data files (only those under the C:\Users folder though), but will not retain your existing Windows settings, or any application installs.
Clean Install: This type of install can be run from the Windows Upgrade Assistant, or from a physical copy of Windows 8. This type of install will wipe your system out and not retain any of your data. Be sure that you have everything you need backed up before starting this type of install.
If you run the Windows Upgrade Assistant to obtain your copy of Windows 8, you will see different labels for the different options before the upgrade gets underway. Here are the equivalent terms within the Upgrade Assistant window at the
Choose what to keep section:
In-Place Upgrade = "Keep Windows settings, personal files, and applications"
Migration Install = "Keep personal files only" or "Keep Windows settings and personal files"
Clean Install = "Nothing"
Upgrade Scenarios:
From versions prior to Windows XP: Not supported. Your only option here is to perform a clean install of Windows 8. Be sure to back up any important data before you begin the setup process.
From Windows XP: Upgrades to Windows 8 are possible, but are limited to Migration installs only. Only personal files can be moved. Refer to the Windows 8 Upgrade paths link above.
From Windows Vista: Upgrades to Windows 8 are possible, but are limited to Migration installs only. Personal files and some Windows settings can be moved. Refer to the Windows 8 Upgrade paths link above.
From Windows 7: In-Place upgrades to Windows 8 are possible. Refer to the Windows 8 Upgrade paths link above.
4b. Q) What about 32 bit to 64 bit upgrades?
A) This is not possible. Upgrading from an existing 32 bit copy of Windows to a 64 bit copy of Windows 8 or Windows 8 Pro requires you to backup your data, wipe the drive clean, and do a completely fresh install of Windows 8 (in fact, this was the case with 64 bit versions of Windows XP, Vista, and 7 as well). In addition, when you run the Windows Upgrade Assistant, the version of Windows you will be upgrading to is the same architecture as the one you are currently running. In the simplest terms, this means if you have a 32 bit version of Windows XP, Vista, or 7 and you run the Upgrade Assistant (even if you choose the Clean Install option), you will end up with a 32 bit version of Windows 8. This works exactly the same for existing 64 bit installs of XP, Vista and 7. If you have a 32 bit copy of a previous Windows version now, and want to move to a 64 bit copy of Windows 8, you need some physical medai with the Windows 8 install disk on it. This can either be a DVD, or a USB flash drive.
Anyone claiming to be able to do a cross-architecture "upgrade" without wiping the disk out either has no idea what they are talking about, or is outright lying.
4c. Q) Can I do a clean install of Windows 8 from an Upgrade disk if I do not have an existing copy of Windows on my computer. For example, if I am using some flavor of Linux, or my hard drive is blank?
A) Unfortunately, no. While you can technically perform said clean install using an upgrade disk, that copy of Windows 8 will never activate, and you will have to completely wipe the machine out and start over. At Microsoft's website, they spell it out pretty clearly:
If you see error 0xC004F061 when you try to activate Windows 8, it means that you're using a product key for an upgrade version of Windows 8 and a previous version of Windows wasn't on your PC when Windows 8 was installed. To install an upgrade version of Windows 8, you must already have Windows 7, Windows Vista, or Windows XP installed on your PC.
If you formatted the drive before the upgrade version of Windows 8 was installed, you won't be able to use your upgrade product key to activate Windows 8. To activate Windows 8, you'll need to install your previous version of Windows, and then reinstall Windows 8.
Source:
http://windows.microsoft.com/en-US/windows-8/why-activate-windows